Patent Application
20230058735
The present invention relates to an accelerator and a particle therapy system, and particularly to a technique for supplying ions to an accelerator.
Particle therapy in which a target volume is irradiated with a particle beam is widely performed. In general, a particle therapy system including an accelerator is used in particle therapy. Ions such as carbon ions, helium ions, and protons are injected into the accelerator, and the ions are accelerated until the ions have energy necessary for therapy. The particle therapy system irradiates the target volume with a charged particle beam by the ions accelerated by the accelerator. In the particle therapy system, the energy and spatial spreading of the charged particle beam is adjusted in accordance with a position and a shape of the target volume.
The accelerator includes a cyclotron that accelerates ions injected into a magnetic field by a radiofrequency electric field and causes the ions to travel in a container. In the cyclotron, the ions travel while increasing a trajectory radius together with acceleration. The energy of the ions increases as the trajectory radius increases, and the ions are extracted when the energy reaches the maximum energy. In the cyclotron, since the ions are extracted when the energy reaches the maximum energy, it is difficult to control the energy of the charged particle beam.
The accelerator also includes a synchrocyclotron described in PTL 1 and NPL 1 below. Unlike the cyclotron in which the ions are accelerated by the radiofrequency electric field having a constant frequency, a frequency of the radiofrequency electric field is modulated according to a change in motion period caused by a mass change of the ions in the synchrocyclotron. Due to this frequency modulation, the ions are accelerated when the ions pass through a region where a radiofrequency electric field is generated. In the synchrocyclotron, it is difficult to control the energy of the charged particle beam to be extracted for the same reason as the cyclotron.
PTL 2 and PTL 3 below describe an eccentric trajectory type accelerator in which energy of extracted ions can be controlled. In a general cyclotron, an ion injection port is provided at a center of an upper surface of a container in which the ions travels, and an ion extraction port is provided on a side surface of the container.
On the other hand, in the eccentric trajectory type accelerator, the ion injection port is provided at a position shifted toward the beam extraction port side from the center of the upper surface of the container. As a result, a plurality of beam closed trajectories around which ions having different energies travel around become dense on the beam extraction port side. Accordingly, in the vicinity of the beam extraction port, it is easy to separate the ions having different energies from the beam closed trajectories, and charged particle beams having different energies are efficiently acquired. Note that, NPL 2 describes a technique of injecting ions from an outside of an accelerator.
PTL 1: JP2013-541170A
PTL 2: JP2019-96404A
PTL 3: WO2016/092621A
NPL 1: W. Kleeven, “The IBA Superconducting Synchrocyclotron Project S2C2”, Proceedings of Cyclotrons 2013
NPL 2: P. Mandrillon, Injection into cyclotrons, CAS, CERN 96-02, (1996), p. 153.
A tubular through-hole extending from the upper surface to the internal space is formed in the container constituting the eccentric trajectory type accelerator, and an opening of the through-hole in the upper surface of the container is the ion injection port. A coil is provided in the container, and the coil generates a magnetic field in the container to deflect the ions. The magnetic field generated by the coil passes through the through-hole. Thus, when the ions are injected into the through-hole, the ions may be deflected by the magnetic field passing through the through-hole, and the ions may collide with an inner wall of the through-hole and may disappear.
An object of the present invention is to prevent disappearance of ions supplied to an eccentric trajectory type accelerator.
The present invention includes a laser source and a target that emits ions by being irradiated with a laser beam emitted from the laser source, and accelerates the ions emitted from the target.
According to the present invention, the disappearance of the ions supplied to the accelerator can be prevented.
An embodiment of the present invention will be described with reference to the drawings. The same components illustrated in the plurality of drawings are denoted by the same reference signs, and the description thereof is simplified. Terms representing shapes such as “columns” in the present specification do not indicate only geometrically strictly defined shapes. The terms representing the shapes in the present specification also indicate shapes to which deformation is applied within a range in which a function of the component can be secured.
An upper portion which is a portion on an upper side of the container 10 and a lower portion which is a portion on a lower side of the container have container shapes in which a cylindrical upper end and a cylindrical lower end are closed, respectively. The upper portion and the lower portion are joined with openings facing each other, and an internal space is evacuated. As will be described later, each of the upper portion and the lower portion is a magnet that generates a magnetic field in the internal space of the container 10.
A laser source 12 is provided on an upper surface of the upper portion. The laser source 12 may be, for example, a carbon dioxide laser, a helium neon laser, a YAG laser, a titanium sapphire laser, or the like. A laser injection through-hole 14 extending from an outside of the container 10 to the internal space is provided at a position where the laser source 12 is provided in the upper portion. A target 20 is disposed at an ion injection point 18 is present at a position immediately below the laser injection through-hole 14.
The target 20 is made of a material that generates ions by being irradiated with a laser beam. The target 20 may be made of a material containing carbon. The target 20 may be formed by using, for example, a metal plate on which carbon is deposited.
The laser source 12 generates a laser beam 16 toward the laser injection through-hole 14. The target 20 is irradiated with the laser beam 16 via the laser injection through-hole 14. When the intensity of the laser beam 16 focused on the target 20 is sufficient to ionize atoms constituting the target 20, plasma is generated. As will be described later, the eccentric trajectory type accelerator 1 includes an ion generator including the target 20. The ion generator supplies ions to the ion injection point 18 by separating electrons and ions from the plasma.
Annular main coils 38 are arranged between the upper magnetic pole 62 U and the yoke 36 and between the lower magnetic pole 62 L and the yoke 36, respectively. The upper and lower main coils 38 travel around an upper end portion and a lower end portion of the columnar space inside the container 10. Each of the main coils 38 is a superconducting coil. A cryostat 60 is disposed around each main coil 38, and the cryostat 60 cools each main coil 38. The upper main coil 38 and the upper portion of the container 10 constitute an upper magnet, and the lower main coil 38 and the lower portion of the container 10 constitute a lower magnet.
The upper magnetic pole 62U and the lower magnetic pole 62L face each other, and a region sandwiched between the magnetic poles 62U and 62L includes an ion passage region 61 through which ions pass while traveling. Each of the main coils 38 generates a magnetic field in an axial direction of the columnar space, and generates a magnetic field passing through the magnetic poles 62U and 62L and the ion passage region 61 in a vertical direction.
As illustrated in
The eccentric trajectory type accelerator 1 includes an acceleration radiofrequency power supply 26, the radiofrequency acceleration cavity 22, a dee electrode 48, and a dummy dee electrode 50. The acceleration radiofrequency power supply 26 excites a radiofrequency magnetic field in the radiofrequency acceleration cavity 22. The radiofrequency acceleration cavity 22 generates a radiofrequency electric field for accelerating ions in the dee electrode 48. The radiofrequency acceleration cavity 22 includes a variable capacitor 24 for modulating a frequency of the radiofrequency electric field by changing its own resonance frequency, and a motor 34 for changing an electrostatic capacitance of the variable capacitor 24.
The cavity upper wall 48A, the cavity lower wall 48B, and the cavity sidewall 48C constitute the dee electrode 48. The dummy dee electrode 50 is formed by an annular conductor surrounding a region facing a dee electrode opening 48E formed by the cavity upper wall 48A, the cavity lower wall 48B, and the cavity sidewall 48C. That is, the dummy dee electrode 50 has an annular conductor facing an edge of the dee electrode opening 48E. An acceleration gap 52 is formed between the dummy dee electrode 50 and the cavity upper wall 48A, the cavity lower wall 48B, and the cavity sidewall 48C.
In this manner, the dee electrode 48 forms a spread-out cavity space surrounded by the cavity upper wall 48A, the cavity lower wall 48B, and the cavity sidewall 48C. The dee electrode 48 has the dee electrode opening 48E that spreads in two different directions as viewed from a base side where the spread-out cavity space spreads. The ion injection point 18 is present between a bent portion of the dummy dee electrode 50 and the dee electrode opening 48E. The ion generator 64 is positioned between the bent portion of the dummy dee electrode 50 and the dee electrode 48, and supplies ions to the ion injection point 18 by irradiating an internal target with a laser beam.
The electrostatic capacitance of the variable capacitor 24 is changed by the rotation of the motor 34, and a resonance frequency of the radiofrequency acceleration cavity 22 is changed (
The ions pass through the spread-out cavity space sandwiched between the cavity upper wall 48A and the cavity lower wall 48B in an arc, pass through the acceleration gap 52 on a left side, and pass under a left side of the dummy dee electrode 50. The ions further pass through a near side of the dummy dee electrode 50 in an arc, pass through a right side of the dummy dee electrode 50, pass through the acceleration gap 52 on a right side, and return to the spread-out cavity space sandwiched between the cavity upper wall 48A and the cavity lower wall 48B. The ions travel around such a trajectory while increasing a trajectory radius, are warped outward by an action of an electric field and a magnetic field to be described later, pass through a high energy beam transport system 32 provided in the beam through-hole 30, and reach an outside of the eccentric trajectory type accelerator 1.
The ions travel around while increasing the trajectory radius so as to approach a maximum energy trajectory 54 indicated by a broken line in
An extraction septum magnet 42 is provided outside the disturbance electrode 46. The kick magnetic field generating shim 44, the disturbance electrode 46, and the extraction septum magnet 42 are used to extract the charged particle beam outward from the eccentric trajectory type accelerator 1.
A radiofrequency voltage is applied to the disturbance electrode 46, and thus, a disturbance electric field is generated from the disturbance electrode 46. The disturbance electric field kicks traveling ions in a direction along a trajectory plane, and causes the ions to depart from the designed trajectory. The ions of which the trajectory departs from a design trajectory pass near the kick magnetic field generating shim 44. The magnetic field generated by the kick magnetic field generating shim 44 restricts a stable region with respect to traveling ions, and introduces ions that exit from the stable region into the extraction septum magnet 42. The ions introduced into the extraction septum magnet 42 pass through the high energy beam transport system 32, and are extracted to the outside of the eccentric trajectory type accelerator 1.
In the eccentric trajectory type accelerator 1, the ion generator 64 including the target 20 is provided at a position away from a central axis of the columnar space. Accordingly, the ion injection point 18 is positioned at a position away from the central axis of the columnar space. A position of the ion injection point 18, that is, a position of the target 20 is a position away from a center of a trajectory of an ion having certain energy.
As a result, a plurality of beam closed trajectories in which ions having different energies travel become denser on a side of the extraction septum magnet 42 than the central axis of the columnar space. Thus, in the vicinity of the extraction septum magnet 42, it becomes easy to separate the ions having different energies from the beam closed trajectories, and charged particle beams having different energies are efficiently acquired.
Referring back to
The laser beam 16 injected into the housing 66 is focused on the target 20 by the condenser lens 71. The condenser lens 71 may be a parabolic mirror or an off-axis parabolic mirror. An optical device such as a mirror may be installed to cause the laser beam 16 to pass around.
When the focused laser beam 16 has an intensity sufficient to ionize atoms constituting the target 20, plasma 72 is generated. The focusing intensity may be equal to or greater than 108 W/cm2 and equal to or less than 1014 W/cm2. The focusing intensity may be changed according to the required amount of charges of ions. The material forming the target 20 may be changed according to the type of ions to be generated. The material of the target 20 may be any of a solid, a liquid, and a gas. When the target 20 is liquid or gas, the amount of charges of ions can be changed by changing a volume.
A plurality of targets made of different materials may be provided in the housing 66. In this case, a plurality of types of ions having different masses or valences emitted from the plurality of targets may be accelerated by the eccentric trajectory type accelerator 1. The valence of the ions to be generated may be switched by changing an irradiation time of the laser beam 16.
A radiofrequency electric field is generated between the housing 66 and the dee electrode 48 by the AC power supply 25. Ions 74 are separated from the plasma 72 by a radiofrequency electric field generated between the housing 66 and the dee electrode 48, and the ions 74 are extracted through the ion extraction hole 70. Note that, the ions 74 may be extracted by using an electrostatic field instead of the radiofrequency electric field. In this case, a DC power supply is connected between the housing 66 and the dee electrode 48, and a DC voltage is applied between the housing 66 and the dee electrode 48.
When the ion generator 64 is irradiated with the laser beam 16 in a shape of a pulse waveform, the amount of charges of ions and a beam time width are controlled by controlling a frequency and a pulse width when the laser beam 16 is generated in a shape of a pulse waveform, for example. Here, the beam time width is defined as a duration in which the charged particle beam is extracted from the eccentric trajectory type accelerator 1. The amount of charges of ions and the beam time width can also be controlled by adjusting a phase of the radiofrequency electric field and an ion generation timing. By such control, a charged particle beam having any amount of charges and any beam time width is extracted from the eccentric trajectory type accelerator 1 at any timing.
Note that, although the eccentric trajectory type accelerator 1 including one laser source 12 has been described above, a plurality of laser sources that irradiate one target with the laser beam may be provided. In this case, a laser injection through-hole may be individually provided for each of the plurality of laser sources. The focusing intensity on the target increases by using the plurality of laser sources.
In the eccentric trajectory type accelerator 1 according to the present embodiment, the target 20 is provided at the ion injection point 18, and the target 20 is irradiated with the laser beam. As a result, the ions are supplied to the ion injection point 18. Accordingly, the disappearance of the ions due to the collision of the ion with an inner wall of the through-hole before the ions are supplied into the accelerator as in the related art in which the ions pass through the through-hole provided in the container is avoided.
The eccentric trajectory type accelerator 2 according to the present embodiment is different from the eccentric trajectory type accelerator 2 illustrated in
Here, although the configuration in which the laser source 12 is provided on the side surface of the container 10 has been illustrated, the laser source 12 may be installed on any surface of the container 10. The laser injection through-hole 14 does not necessarily extend in a direction orthogonal to the surfaces of the ceiling part 10U and the yoke 36. An optical path of the laser beam 16 may be formed by using any optical device such as an optical lens or an optical mirror. The optical path of the laser beam 16 may be any optical path from the laser source 12 to the ion generator 64.
In the eccentric trajectory type accelerator 2 according to the second embodiment, the disappearance of the ions due to the collision with a wall surface of the through-hole before the ions are supplied to the ion injection point 18 is avoided by the principle similar to that of the first embodiment.
As illustrated in
The eccentric trajectory type accelerator 4 is the eccentric trajectory type accelerator according to the first embodiment or the second embodiment. The eccentric trajectory type accelerator 4 accelerates the ions forming the charged particle beam. The beam transport device 90 transports the charged particle beam accelerated by the eccentric trajectory type accelerator 4 to the irradiation nozzle 92. The irradiation nozzle 92 irradiates the target volume in the patient 100 fixed to the couch 101 with the charged particle beam transported by the beam transport device 90. The irradiation nozzle 92 shapes the charged particle beam according to a shape of the target volume, and irradiates each irradiation spot in the target volume with the shaped charged particle beam.
The irradiation nozzle 92 includes a dose monitor, and measures an irradiation dose for each irradiation spot. Based on the measured value obtained in this manner, the irradiation control section 94 calculates a required dose for each irradiation spot. The irradiation control section 94 outputs the required dose for each irradiation spot to the accelerator control section 96. The accelerator control section 96 controls the energy, the extraction timing, and the like of the charged particle beam in the eccentric trajectory type accelerator 4 based on the required dose.
Note that, the beam transport device 90 of the particle therapy system 3 is not limited to a fixed beam transport device. The beam transport device 90 may be a transport system that is rotatable around the patient 100 together with the irradiation nozzle 92 called a rotating gantry. The number of irradiation nozzles 92 is not limited to one, and a plurality of irradiation nozzles may be provided. The form of the particle therapy system 3 may be a form in which the beam transport device 90 is not provided and the charged particle beam is directly transported from the eccentric trajectory type accelerator 4 to the irradiation nozzle 92.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-053547 | Mar 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/031754 | 8/24/2020 | WO |
